Most types of optical scanners that are used in laser beam printers and the like have a configuration including a semiconductor laser as a light source, a polygon mirror that is a rotary polyhedral mirror, a first image forming optical system for linearly focusing a light beam from the light source on the rotary polyhedral mirror so that a tilt of a surface of the rotary polyhedral mirror can be corrected, and a second image forming optical system for forming a uniform beam spot on a surface to be scanned at a constant speed.
The conventional optical scanners have been of a configuration in which a second image forming optical system is referred to as an fθ lens and formed of a plurality of large-sized glass lenses. Because of this, the second image forming optical system hardly is reduced in size and is costly, which has been disadvantageous.
With this as a background, in recent years, for the realization of size and cost reductions, a configuration in which a single curved surface mirror is used in a second image forming optical system has been proposed as in JP 4(1992)-194814 A, JP 6(1994)-118325 A, JP 6(1994)-281872 A, JP 6(1994)-281873 A, JP11(1999)-30710 A, JP11(1999)-153764 A and the like.
Meanwhile, in order to achieve high-speed scanning by increasing a rotation speed of a rotary polyhedral mirror, JP2001-33725 A or the like proposes a configuration regarding a bearing for a rotary shaft that withstands high-speed rotation. Further, JP2000-19443 A or the like proposes a configuration in which a rotary polyhedral mirror with an increased number of reflecting surfaces is used so that scanning can be preformed at a higher speed without increasing a rotation speed.
However, in most types of optical scanners of the configuration including a single curved surface mirror, while excellent performance can be attained in terms of a field curvature, fθ characteristics and a scanning line curvature, an excellent beam spot cannot be obtained due to insufficient correction for light ray aberration, which has been disadvantageous.
Furthermore, JP11(1999)-30710 A proposes an optical scanner and describes a configuration as a model in which the optical scanner leads a light beam from a curved surface mirror directly to an image plane. However, in this configuration, a light beam is reflected off the curved surface mirror at a small reflection angle, and thus in practice, in order for the light beam to be led to a photosensitive drum, it is required that a returning mirror be disposed between the curved surface mirror and the photosensitive drum. Further, a cross section in a sub scanning direction of the curved surface mirror is not in a circular arc shape but in a shape represented by a four-dimensional polynomial, thereby making it difficult to perform processing and evaluations with respect to the curved surface mirror, which has been disadvantageous.
In a configuration disclosed in JP11(1999)-153764 A, a light beam from a single curved surface mirror can be led directly to a photosensitive drum without requiring a returning mirror, and the curved surface mirror is formed in a shape that allows processing and evaluations to be performed with relative ease. However, in this configuration, light beams from two light sources are scanned so that scanning can be performed at a higher speed, thereby increasing the number of constituent components, which has been disadvantageous.
In the configuration proposed in JP2001-33725 A, a bearing part of a rotary polyhedral mirror is formed of a dynamic pressure bearing so as to withstand high-speed rotation. However, in this configuration, it is required that the bearing part be processed with high accuracy, thereby incurring higher cost, which has been disadvantageous.
In the configuration proposed in JP 2000-19443 A that uses the rotary polyhedral mirror with an increased number of reflecting surfaces, a second image forming optical system is formed of a single lens. However, in this configuration, since the single lens is configured as an aspheric lens, it is required industrially that the lens be formed of a plastic lens, thereby increasing the change in performance according to temperature characteristics, which has been disadvantageous.